Hi Bill - I have a 88 560sel. Had rough idle ( just slight shake - the odd miss) for two years. I finally found it!! Take the injector lines off the fuel distributor and VERY GENTLY remove the fine screens in the fuel distributor that filter dirt from going to each injector. What I used was a thin long screw with a very sharp point and turned it slightly to just grab the screen to be able to pull it out, being careful not to make a big hole in the screen. You could buy new screens - see if Phil has them here - for the price it might make it easier for you. If you see dirt on the screens - you have found the problem. I would clean mine and the car would idle smoothly for about 2 weeks and then start missing again. Problem??? Rusted fuel lines from the tank. The fuel filter is in BACK of the car so it cannot catch the rust as it comes off the inside of the lines. The last stop before injectors is the screen filters in the fuel distributor. When I took my lines out (pain in butt at back by fuel tank) -one broke in two from it's own weight. New lines = no more rough idle. At idle there was enough dirt on the screen to slow the fuel delivery causing the roughness--when you stepped on the gas it worked OK injector cleaners helped very little. Hope this helps you.